Mark paint/markers/ellipsis-mixed-text-in-ltr-flow-with-markers.html flaky

This test is flaky on macOS.

Bug: 978966
Change-Id: I2a5e9e387ba7870b2e8a6c5516c64aeb1dea0bee
Tbr: pdr@chromium.org
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678850Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672620}

third_party/blink/web_tests/TestExpectations

@@ -6209,3 +6209,6 @@ crbug.com/974675 http/tests/devtools/sources/debugger-frameworks/frameworks-jque
 crbug.com/967927 http/tests/devtools/startup/console-clear-arguments-on-frame-navigation.js [ Skip ]
 crbug.com/967927 http/tests/devtools/startup/console-clear-arguments-on-frame-remove.js [ Skip ]
 crbug.com/967927 http/tests/devtools/startup/console-log-before-frame-navigation.js [ Skip ]
+
+# Sheriff 2019-06-26
+crbug.com/978966 [ Mac ] paint/markers/ellipsis-mixed-text-in-ltr-flow-with-markers.html [ Pass Failure ]